Argumentation is the process of giving arguments, counter-arguments, counter-counter-arguments and so on. In artificial intelligence, since the late 1980s there has been research that tries to use formal methods to approach argumentation theory. One very young approach in formal argumentation theory is to study argumentation under the framework of modal logic. Two possible advantages of doing so are: (1) this is a new angle, from which to look at argumentation we might have new insights, and (2) techniques in modal logic might become available for studying argumentation. In this talk, I would like to provide a different way of connecting argumentation theory and modal logic. More specifically, I would like to try to locate formal argumentation theory within the framework of justification logic. The resulting logic is a justification logic for argumentation.

A cut $I$ in a model $M$ of PA is pseudostandrd if there is an $N$ such that $(M,I)$ is elementary
equivalent to $(N,omega)$. I will discuss some preliminary results in model theory of pseudostandard cuts.